1. Field of the Invention
The invention relates to the field of lighting panels and in particular to panels for providing directed polarized illumination.
2. Description of the Prior Art
It has been appreciated by some researchers that visual effectiveness and visual acuity are a function both of the quality of the illumination employed to observe objects and detail as well as the intensity of illumination. In particular, it has been appreciated that the contrasts of observed objects are significantly more important than the intensity of the illumination to reflect things. For example, visual effectiveness is improved as much by a one percent increase in observed contrasts of objects as by an increase of fifteen percent of foot candle illumination intensity of the object. See H. Richard Blackwell, "A General Quantitative Method for Evaluating the Visual Significance of Reflected Glare, Utilizing Visual Performance Data", I.E.R.I. Project No. 70, LIGHTING, W.R.C. Smith Publishing Company.
It has also been appreciated by some that the reduction in the amount of direct glare at wide angles from the vertical from a source of illumination is increased more by multilayer polarizing panels than by conventional prismatic acrylic panels, glass diffuser panels or various type of light contoured panels. See Blackwell, supra.
It has also been determined by prior practitioners that in order to minimize glare reflected from on object, it is highly desirable to have light projected in a conical annulus defined between an inner cone at approximately thirty degrees from the vertical from an overhead light source to approximately sixty degrees from the vertical. Light impinging on an object, such as printed matter, above and slightly forward of a reader results in a considerable amount of reflection both from the black printed surface as well as the white surrounding paper. Thus, the normal contrast of printed matter is somewhat reduced and in the case of an extremely high illumination level, the printed matter may be totally unreadable. If the printed matter is illuminated from the side, direct reflection from the surface of the printed matter will not be directed to the eyes of the reader, but instead will be reflected away from the observer and, some of the light will be scattered and not reflected by the printed page. Some of the scattered light will be directed to the eye of the observer and light scattered from the white paper will be considerably different than light scattered from the black print and the contrast between the print and the paper will be largely undiminished.
However, in office or school settings where a ceiling is provided with a plurality of illumination panels, the lighting industry has devised a standard for visual comfort probability (VCP) which is a measure of the contribution of the various light sources to the subjective feeling of visual comfort or discomfort in the room. VCP is based upon investigations performed to find the borderline between between comfort and discomfort, which might be interpreted as the point at which fifty percent of the people in the room consider it comfortable and the other fifty percent do not. The worst position in the room is typically used to calculate the VCP in a room. The Illuminating Engineering Society has set a VCP of 70 as a practical guide to a minimum comfort level. Thus, VCP is a percentage of people who, when viewing from a specified location and in a specified direction, will be expected to find it acceptable in terms of discomfort glare from the light source.
It has been appreciated that the VCP which is required within buildings under arrays of illuminations panels can be better met if the illumination from each panel at high angles from the vertical is reduced, particularly if minimized at angles above sixty degrees from the vertical. JONES, "Lighting Panel", U.S. Pat. No. 3,829,680 (1974). A panel has been sold under the trademark, Radialens. This product produces maximum wide angle ditribution within rooms in all directions, but does not meet VCP's of 70 or more.
It has also been appreciated by some practitioners that when an object is viewed at any angle, a ray of light coming from a point in space will produce higher contrast when vertically polarized than when horizontally polarized. See Blackwell, supra. For this reason methodologies and materials have been developed for producing polarizing sheets as described in greater detail in KAHN, "Light Polarizing Structures", U.S. Pat. No. 3,124,639 (1964) and KAHN et al., "Machine for Continuously Producing Large Area Light Polarizing Panels", U.S. Pat. No. 3,772,128 (1973), which is incorporated herein by reference. These panels meet the VCP of 70 or more due to their low brightness polarizing effect at wide angles.
Therefore, the applicant has combined the prismatic structure of the JONES Radialens manufactured according to the methodologies and structures of KAHN's polarizing panel to obtain the higher visual acuity realized with polarized light with the wide angular light distribution of the Radialens panel and the desirable VCP of 70 or more into an integral structure. However, what is achieved is not merely the expected advantages of polarized and Radialens light distribution, but a marked improvement in visual comfort probability (VCP) due to a reduction of brightness and glare at wide angles.